1. Field of the Invention
The present invention relates to a reversible thermal recording medium and a recording method using this recording medium.
2. Description of the Related Art
Recently, with the advance of office automation the amount of various information has significantly increased, and chances of information output have also increased with increasing information amount. Generally, information outputs are classified into a hard copy output from a printer to paper and a display output. Unfortunately, in the hard copy output a large quantity of paper is consumed as a recording medium if the information output amount increases. Therefore, the hard copy output is expected to be a problem in the future in respect of natural resource protection. On the other hand, the display output requires a large-scale circuit board in a display unit. This brings about problems of portability and cost. For these reasons, a rewritable recording medium capable of reversibly recording and erasing display images, which is free from the above conventional problems, is anticipated as a third recording medium.
Conventionally, as a recording material of such a rewritable recording medium, a composition containing a color former, e.g., a leuco dye, and a developer, e.g., an acid, has been extensively studied. This composition develops or loses a color in accordance with the interaction between the color former and the developer. For example, as a recording medium capable of chemically repeating coloring and decoloring when supplied with thermal energy, Jpn. Pat. Appln. KOKAI Publication No. 4-50,290 has proposed a composition consisting of a leuco dye, an acid as a developer, and a long-chain amine as a decolorizing agent. Also, it is reported in the 42nd Polymer Forum Preprints, 1993, page 2,736, and Jpn. Pat. Appln. KOKAI Publication Nos. 4-247,985, 4-308,790 and 4-344,287 that in a composition prepared by mixing a leuco dye and long-chain alkyl phosphonic acid, coloring and decoloring occur reversibly when the crystal forms are altered under control using thermal energy. Another conventional recording material is described in Japan Hardcopy '93, pp. 413 to 416. This recording material makes use of the fact that in a composition system consisting of a leuco dye which is highly amorphous and a long-chain 4-hydroxyanilide compound which is highly crystallizable, reversible coloring and decoloring based on a crystalline-to-amorphous transition of the entire composition system take place under control using thermal energy.
These recording materials, however, are generally unsatisfactory in colorlessness in the decolored state, so the contrast ratio between the colored and decolored states obtained by these materials is not so high. In particular, these recording materials have the tendency that the background display is hard to provide because it is difficult to obtain a colorless, transparent state. In the composition system as discussed above in which the long-chain 4-hydroxyanilide compound is blended as a developer, the contrast ratio is relatively high, but a large thermal energy is required in melting the crystal in the crystalline-to-amorphous transition of the composition system. This is a disadvantage in respect of energy savings. Another example of the material which changes the colored state in the crystalline-to-amorphous transition is an Ni complex disclosed in Mol. Cryst. Liquid Cryst. 1993, 235, p. 147. This material develops green color in the crystalline state and red color in the amorphous state and is neither colorless nor white in either of the crystalline or amorphous state. Therefore, by using this material it is difficult to realize a display with a high contrast ratio.
As discussed above, many attempts have been conventionally made to use a composition system containing a color former and a developer, as the recording material for a rewritable recording medium. Unfortunately, none of these composition systems has been put into practical use due to problems of, e.g., low contrast ratio between the colored and decolored states and energy savings.
In addition, as rewritable recording media capable of recording and erasing by using a thermal printer head (TPH), composition systems comprising an organic low molecular weight compound and a high molecular weight resin matrix are disclosed, for example, in Jpn. Pat. Appln. KOKAI Publication Nos. 55-154,198 and 57-82,086, which have been employed in some prepaid cards. The composition systems, however, have disadvantages that their operable temperature range is very narrow, in which recording and erasing can be performed in a short time by using TPH, and that their repeatable times between recording and erasing are limited to about 150 to 500 times. Consequently, the application fields of the rewritable recording media are greatly restricted. Therefore, it is difficult to apply them to a card for station service where the operating temperature range is very wide. Moreover, the composition systems have a disadvantage that they are poor in visibility due to a reversible change between a cloudy state and a transparent state.